With regard to the aerodynamic properties of axial flow compressor and turbine stages, it is crucial that a small and as constant as possible radial gap be maintained between the blade tips and the outer flow channel wall. This requires most importantly that the wall structure exhibit sufficient dimensional stability and geometric precision. It is critical that the geometry be altered as little as possible by thermal and mechanical influences. The mostly hot working gas should essentially act only on the interior side of the structure; losses due to leakage through the structure must be minimized. In steady-state operation, it is advantageous when the mostly thermally induced dimensional variation of the wall structure is adapted temporally and in terms of magnitude to that of the bladed rotor. Since mechanical contacting between the blade tips and the wall structure is hardly avoidable under certain applications of force, the interior side of the wall structure should be designed to be deformable, resilient and, accordingly, to have run-in capability, at least on the blade-tip side.
The German patent DE 100 20 673 C2 describes a ring structure of metal construction for the rotor blade region of axial flow compressor and turbine stages. The ring structure it discusses has an annular outer wall, which is designed as a closed, mechanically stable housing wall of the compressor or turbine stage. The ring structure it discusses also has an annular inner wall, as well as a connecting structure designed as a hollow-chamber structure, the connecting structure designed as a hollow-chamber structure being positioned in a sandwich-type arrangement between the outer wall and the inner wall. On one side, the connecting structure designed as a hollow-chamber structure is connected to the outer wall and, on the other side, to the inner wall. The annular chamber structure in accordance with German patent DE 100 20 673 C2 has an inner wall, which is interrupted over its circumference by a plurality of axially or primarily axially extending expansion joints. An inner wall segmented in this manner by expansion joints has the disadvantage that flow losses can occur. In addition, the complexity of the ring structure and thus the assembly as well as manufacturing outlay are increased by an inner wall segmented in this manner. Moreover, during operation, chipping off, washout or damage due to erosion can occur at the edges of the expansion joints, further increasing the flow losses.